With the winter cold and flu season upon us, Americans should be on the lookout for another ultra-contagious virus: our most common stomach bug.
The U.S. experienced the largest December norovirus surge since at least 2012, according to data from the Centers for Disease Control and Prevention.
During the week of Dec. 5, state health departments recorded 91 separate outbreaks nationwide, according to the CDC. The next highest figure for that week since 2012 was 65 outbreaks.
Through November, there have been 55 laboratory-confirmed cases of norovirus in California, according to the state department of public health. Data from December are not yet available.
Outbreaks are typically more widespread in January and February, the data show, raising concerns that the surge could continue.
Most norovirus cases are transmitted directly from one person to another, the CDC said, from actions such as touching food or eating utensils. Restaurants, cruise ships, healthcare facilities and schools are common transmission sites.
Additionally, contaminated food, water and surfaces can spread the virus.
Last week, the U.S. Food and Drug Administration issued advisories warning businesses and consumers against serving or eating oysters from Washington state, Canada and South Korea which may be contaminated with norovirus.
Annually, the CDC reports some 2,500 outbreaks nationally. But real-time monitoring data cover only 14 states: Alabama, Colorado, Massachusetts, Michigan, Minnesota, Nebraska, New Mexico, North Carolina, Ohio, Oregon, South Carolina, Tennessee, Virginia and Wisconsin.
Around half of outbreaks of food-related illness are caused by norovirus, per the CDC.
The most common symptoms of norovirus are vomiting, diarrhea, nausea and stomach pain. Symptoms usually improve in one to three days, but those infected can still spread the virus for several days after symptoms ease.
The best ways to prevent the virus from spreading: washing hands, cooking shellfish thoroughly, washing fruits and vegetables, cleaning and disinfecting contaminated surfaces, washing laundry in hot water and staying home for two days after symptoms stop.
The main treatment for norovirus is hydration to replace fluids lost by the patient. Those with severe dehydration should seek medical attention, the CDC advises.